feat: transients, unchecked arithmetic, hash helpers + fix REPL set printer

More clojure.core coverage (real implementations, not stubs):
- transient/persistent!/conj!/assoc!/dissoc!/pop! as correctness-preserving
  aliases (Jolt collections are persistent, so no in-place speedup but correct)
- unchecked-* arithmetic (Jolt numbers don't overflow -> plain ops)
- hash-combine/hash-ordered-coll/hash-unordered-coll (24-bit masked to stay
  in integer range), ex-cause (ex-info now records a cause), prefers, random-uuid

Fix a pre-existing REPL bug: the set printer in main.janet iterated the backing
phm's table keys (:buckets/:cnt) instead of the elements, so every set printed
as #{:buckets :cnt} at the REPL (pr-str was already correct). Now uses phs-seq.

conformance 218/218, features 78/78, jank 120.

src/jolt/main.janet

@@ -96,10 +96,9 @@
     (do
       (push-str buf "#{")
       (var first? true)
-      (each k (keys (v :phm))
-        (when (not= k :jolt/deftype)
-          (if first? (set first? false) (push-str buf " "))
-          (write-value k buf)))
+      (each k (phs-seq v)
+        (if first? (set first? false) (push-str buf " "))
+        (write-value k buf))
       (push-str buf "}"))
 
     (and (table? v) (= :jolt/regex (v :jolt/type)))